The prompt optical flash recently detected accompanying GRB990123 suggests that , for at least some GRBs , \gamma -ray emission is accompanied by prompt optical-UV emission with luminosity L ( 1 - 7.5 { eV } ) \approx 1 \times 10 ^ { 49 } ( \Delta \Omega / 4 \pi ) { erg / s } , where \Delta \Omega is the solid angle into which \gamma -ray and optical-UV emission is beamed .
Such an optical-UV flash can destroy dust in the beam by sublimation out to an appreciable distance , \approx 10 pc , and may clear the dust out of as much as \sim 10 ^ { 7 } ( \Delta \Omega / 4 \pi ) M _ { \odot } of molecular cloud material on an apparent time scale of \sim ten seconds .
Detection of time dependent extinction on this time scale would therefore provide strong constraints on the GRB source environment .
Dust destruction implies that existing , or future , observations of not-heavily-reddened fireballs are not inconsistent with GRBs being associated with star forming regions .
In this case , however , if \gamma -ray emission is highly beamed , the expanding fireball would become reddened on a \sim 1 week time scale .

If the optical depth due to dust beyond \approx 8 pc from the GRB is 0.2 \lesssim \tau _ { V } \lesssim 2 , most of the UV flash energy is converted to infra-red , \lambda \approx 1 \mu m , radiation with luminosity L _ { IR } \approx 10 ^ { 41 } { erg / s } extending over an apparent duration of \approx 20 ( 1 + z ) ( \Delta \Omega / 0.01 ) day .
Dust infra-red emission may already have been observed in GRB970228 and GRB980326 , and may possibly explain their unusual late time behavior .